1. Field of the Invention
The present invention generally relates to a recording medium. More particularly, the present invention relates to a write once recording medium.
2. Description of the Related Art
With the development of the internet and ever increased capacity of computers, more diversified information can be obtained and transferred. The capacity of computers for handling information has gradually evolved from mere data processing in the earlier stage to processing of words, graphs, sounds, static pictures, and to today's processing of high resolution motion pictures. The recording media for storing such information have also evolved from paper in the earlier stage, to magnetic tape and hard disc, and further to today's optical information recording media such as compact disc (CD) and digital versatile disc (DVD).
Optical information recording media have been rapidly brought up to the market due to their advantages of high recording density, small size, long storing period, low manufacturing costs, high compatibility and low error rate in comparison with magnetic tapes and semiconductor memory cells. Among various types of the optical information recording media, the most widely used is write once recording medium (WORM), such as compact disc-recordable (CD-R) and digital versatile disc recordable (DVD-R). In the WORM, organic dye is used to form a recording layer and the information is recorded via deformation of a substrate of the medium induced by structural changes of the organic dye layer under irradiation of a laser beam.
In the foregoing WORM, the organic dyes, which is used as the recording layer, have disadvantages such as high manufacturing costs, difficulty in formulation, complicity of solvent system, sensitivity to laser wavelength, low tolerance to climate (temperature and humidity), short life time and pollution to the environment. Additionally, with respect to the next generation of blue OLE_LINK1laserOLE_LINK1 optical information recording medium, which uses short wavelength (405 nm) blue laser for recording and regenerating, fewer types of organic dyes can be used and, even if used, the organic dyes are more difficult to be coated on the disc substrate via spin coating due to their lower solubility. Furthermore, as a result of significant changes in the disc structure of blue laser optical information recording medium, track pitch (about 0.3˜0.4 μm) on a disc substrate is reduced so that, due to internal tension of organic dye solution, organic dyes cannot be uniformly coated on the disc substrate. Consequently, the coating process can only be performed via evaporation coating, which makes manufacturing processes more difficult.
Therefore, it is desirable to replace organic dyes with inorganic materials as recording layers of WORM. In addition, inorganic materials used as recording layers of WORM can have different material systems corresponding to different writing mechanisms. For example, a prior art (U.S. Pat. No. 5,401,609) discloses a recording method via gas expansion, and the materials used therein are AgOx, FeNx, CuNx, and SnNx. Another prior art (U.S. Pat. No. 4,624,914) discloses formation of recorded marks via structural change by using a mixture of metal and metal oxide. Yet other prior arts (U.S. Pat. No. 4,477,819 and U.S. Pat. No. 5,458,941) disclose formation of recorded marks via interface reaction, and the materials therein are Ge/Al, Si/Al, Al—Cr/Si—Al, and GaSb/Ag. Further, a prior art (U.S. Pat. No. 4,960,680) discloses the formation of recorded marks via irreversible phase change, and the materials therein include Sb—In—Sn and Sn—Sb—Se/Sb—Bi. Comparing to the WORM with organic dyes as recording layers, the WORM with inorganic materials as recording layers has advantages such as simplicity in formation, low pollution to the environment, high light-resistance and high climate-resistance. Thus, the WORM with inorganic materials as recording layers has become the focus of research and development of future recording media.